1. Field of the Invention
This invention relates to aluminum hydroxide having a diminished tendency to cause discoloration when compounded with a plastic composition at an elevated temperature, typically 180° C. In particular, the invention relates to aluminum hydroxide in a plastic composition in which the major polymeric component is a vinyl chloride polymer such as polyvinyl chloride, conveniently abbreviated pvc.
2. Description of the Related Art
For a review of the utilization of aluminum hydroxide as an additive and compounding ingredient in plastic compositions, particularly relating to the use of aluminum hydroxide as a flame retardant, reference can be made to Chapter 81 by Frank Malesky in “Handbook of Plastics Additives and Modifiers”, J. Edenbaum, ed., (New York: Van Nostrand Reinhold 1992), pages 1071-1085.
The problem of imparting to pvc a sufficient heat processing stability at temperatures at which the polymer becomes sufficiently fluid or softened to permit shaping is of long standing. It has been resolved in principle by the addition to the polymer of various combinations of known heat stabilizers. Given the great versatility of plastic compositions in which pvc is the major polymeric component, however, the selection of the right stabilizer or stabilizer combination for any given composition remains an empirical art in which theories and predictions are of limited usefulness.
There exists a prodigiously large literature relating to stabilizers for pvc. For a convenient review and classification reference can be made to Chapters 16-20 in the above-cited “Handbook”, pages 208-337.
The following individual disclosures are believed to represent the closest prior art to the present invention.
M. Tadenuma et al., U.S. Pat. No. 5,004,776 disclosed a thermally stabilized chlorine-containing resin composition comprising a stabilizer consisting essentially of (a) an overbased alkaline earth metal carboxylate or phenolate complex, (b) zeolite, (c) calcium hydroxide and (d) a complex of at least one metal perchlorate selected from the group consisting of sodium, magnesium, calcium and barium perchlorates with at least one compound selected from the group consisting of polyhydric alcohols and their derivatives. The chlorine-containing resin composition may further comprise a known stabilizer for chlorine-containing resins, a known co-stabilizer and other known additives, for example (among others) an inorganic metal salt compound. The inorganic compound includes, for example, magnesium oxide, calcium oxide, calcium phosphate, aluminum hydroxide, synthetic hydrotalcite and the like.
Y. Sato al., U.S. Pat. No. 5,025,051 disclosed a synthetic resin composition comprising (A) 100 parts by weight of a synthetic resin, and (B) 0.01 to 10 parts by weight of a mixed product of (a) a compound having at least one piperidyl group, (b) perchloric acid, and (c) optionally an inorganic substance other than perchloric acid. As the inorganic substance other than perchloric acid, there are d disclosed, for example, an oxide, hydroxide, inorganic acid salt, basic inorganic acid salt or double salt of at least one metal selected from the group consisting of Li, Na, K, Mg, C, Sr, Ba, Zn, Al, Sn and Pb, further a metallic aluminum powder, or a metal oxide such as SnO2, TiO2, or ZrO2. As the oxide of the metal there are mentioned, for example, MgO, CaO, BaO, ZnO, SrO2, or PbO, and as the hydroxide of said metal there are mentioned, for example, LiOH, Mg(OH)2, Ca (OH)2 or Al(OH)3.
K. Bae et al., U.S. Pat. No. 5,034,443 disclosed that a blend of sodium perchlorate hydrate mid calcium silicate provides improved long term heat stability to a polyvinyl chloride resin containing a conventional heat stabilizer, and that a combination of a solution of sodium perchlorate in water with calcium silicate and a non-absorbing diluent powder such as calcium carbonate provides a free-flowing powder composition of reduced hazard level. Other non-absorbing diluent powders include zeolites, silica, alumina, PVC resins, barium sulfate and the like.
Watanabe et al., U.S. Pat. No. 5,190,700 disclosed a flame retardant for a powder halogen-containing vinyl resin which comprises (A) 5 to 40 parts by weight of at least one alkali metal compound of lithium, sodium and potassium calculated as the formula M2O where M is said alkali metal (B) 0.2 to 10 parts by weight of a perchloric acid radical in the form of the acid or a salt or amine thereof, calculated as perchloric acid radical (ClO4), and (C) 1-50 parts by weight of a hydrophobic dispersing agent having a boiling point or a decomposition temperature of 200° C. or higher, based on 100 parts by weight of antimony pentoxide (Sb2O5). Watanabe also acknowledges that a variety of flame retardants have been used, including inorganic materials such as antimony trioxide (Sb2O3), sodium antimonite, aluminum hydroxide, zirconium oxide, zinc borate and borax.
R. Drewes et al., U.S. Pat. Nos. 5,519,077 and 5,543,449 disclosed compositions comprising (a) pvc, (b) perchloric acid or a perchlorate, (c) a terminal epoxide compound and (d) an antioxidant ('077) and (a) flexible pvc, (b) perchloric acid or a perchlorate, (c) a terminal epoxide compound ('443). In each case it is also disclosed that the compositions can contain further additives. These are, for example, fillers and reinforcing materials (for example calcium carbonate, silicates, glass fibers, talc, kaolin, chalk, mica, metal oxides and hydroxides, carbon black or graphite). The fillers used are, for example, chalk, kaolin, china clay, talc, silicates, glass fibers, glass beads, sawdust, mica, metal oxides or hydroxides, carbon black, graphite, rock flour and barytes.
None of the above disclosures mention a tendency to discolor caused by aluminum hydroxide.
Thus, while the well established and successful conventional heat stabilizers provide effective stabilization to the substrate polymer composition whose major polymeric component is pvc at elevated heat processing temperatures during standard processing, they may not provide effective stabilization to additives contained within the polymer (luring such heat processing. For example, die use of aluminum hydroxide in such compositions is not without limitations. It has been found that one important limitation is the tendency of an otherwise adequately stabilized pvc composition to discolor in the presence of aluminum hydroxide, with the intensity of discoloration increasing with increasing use levels of aluminum hydroxide in such composition.
It is therefore a general object of the present invention to provide a plastic composition containing aluminum hydroxide having a diminished tendency to discolor at processing temperatures of the order of 180° C.
It is also an object of the invention to provide an aluminum hydroxide composition having a diminished tendency to discolor when incorporated in a plastic composition whose major polymeric component is pvc.
It is also an object of the invention to provide an aluminum hydroxide composition having enhanced flame retardant properties when incorporated in a plastic composition whose major polymeric component is pvc.
It is a further object of the invention to provide a method of stabilizing a plastic composition whose major polymeric component is pvc and which contains aluminum hydroxide.